1. Field of the Invention
Concrete renovation has grown into a very large field, ever increasing faster and faster, due to the enormous number of reinforced concrete structures built after the second world war. The concrete renovations are necessary as it is extremely difficult to produce concrete structures of such high quality that reinforcement corrosion is avoided. When the reinforcement corrodes, the concrete structure gradually looses its strength. Reinforcement corrosion typically occurs as a result of the destruction of the highly alkaline environment of concrete, e.g., due to cracks in the concrete cover. The cracks are often so fine that they can not be spotted with the naked eye--but are large enough to allow moisture to penetrate and start the corrosion process. Around the cracks, areas emerge emitting iron ions as part of an electric circuit. See FIG. 2. Such an area is the anode of the circuit. Electrons which are freed at the anode are consumed elsewhere along the reinforcement--the cathode of the circuit--thus freeing hydroxide ions (OH--ions). Iron ions and OH--ions together create rust.
2. Brief Discussion of Related Art
For a long time, concrete renovation has consisted of an optical inspection of the surface of the concrete by skilled people, taking out numerous samples of the structure using tubular drills, investigating the samples in laboratories, deciding which areas of the structure must be renewed, breaking up these areas, sand blasting the reinforcement, priming, pre-watering and filling the holes with repair mortar, possibly supplemented with surface treatment of the concrete. The durability of this kind of repair is not well known.
An alternative treatment is cathodic protection. Cathodic protection can only be used if the corrosion is not yet so advanced that the strength of the structure is critical.
Cathodic protection is particularly superior where breaking up the concrete is very inconvenient, i.e., at bridge piers and other structures carrying heavy weight.
Using this method, negative voltage is applied to the reinforcement, binding the positive iron ions to the steel. Typically the reinforcement is held at approximately -0.75 volt compared with the potential of the surrounding concrete. The positive pole of the circuit is typically established using drilled-in anodes or wire netting attached to the surface of the concrete.
In its simplest version, the cathodic protection system consists of a central power supply producing low voltage direct current, a wire connecting the negative supply outlet to the reinforcement, a cable (normally strongly ramified) that leads the positive voltage to the concrete areas to be protected, simple current distributors, and a number of drilled-in anodes. See FIG. 3. Normally, the distributor normally only consists of a series resistor for each anode and maybe one or two jumper selectable common series resistors to lower the common supply voltage and current.
In practice this type of installation has shown too primitive for most structures. The system does not normally allow for individual adjustment of the current to each anode and, more important, there is no immediate way that it can be controlled if the anodes functions as intended.
Presently, the most advanced systems have an over--and undervoltage detector for each anode built into the distributor. All the outputs of the undervoltage detectors are logically ORed and the output of the OR gate is then lead through a separate wire to a control panel near the power supply. The same holds for the overvoltage detectors so that for each distributor two separate wires lead to the control panel where each of them activates a warning light. The system does not tell which anode(s) are malfunctioning only that the malfunctioning anode(s) is(are) connected to a specific distributor.
Furthermore, for large concrete structures an overwhelming number of wires must be taken from the many distributors to the control panel.
Finally the system does not allow for individual adjustments of voltage/current to each anode.